Blood coagulation is the key process involved in both haemostasis (i.e. the prevention of blood loss from a damaged vessel) and thrombosis (i.e. the formation of a blood clot in a blood vessel, sometimes leading to vessel obstruction).
Coagulation is the result of a complex series of enzymatic reactions. One of the ultimate steps in this series of reactions is the conversion of the proenzyme prothrombin to the active enzyme thrombin.
Thrombin is known to play a central role in coagulation. It activates platelets, leading to platelet aggregation, converts fibrinogen into fibrin monomers, which polymerise spontaneously into fibrin polymers, and activates factor XIII, which in turn crosslinks the polymers to form insoluble fibrin. Furthermore, thrombin activates factor V, factor VIII and factor XI leading to a “positive feedback” generation of thrombin from prothrombin.
By inhibiting the aggregation of platelets and the formation and crosslinking of fibrin, effective inhibitors of thrombin would be expected to exhibit antithrombotic activity. In addition, antithrombotic activity would be expected to be enhanced by effective inhibition of the positive feedback mechanism. Indeed, the convincing antithrombotic effects of a thrombin inhibitor in man have been described by S. Schulman et al. in N. Engl. J. Med. 349, 1713-1721 (2003), L. Wallentin et al. in Lancet 362, 789-97 (2003) and H.-C. Diener et al. in Cerebrovasc. Dis. 21, 279-293 (2006).
The early development of low molecular weight inhibitors of thrombin has been described by Claesson in Blood Coagul. Fibrinol. 5, 411 (1994).
Blombäck et al. (in J. Clin. Lab. Invest. 24, suppl. 107, 59 (1969)) reported thrombin inhibitors based on the amino acid sequence situated around the cleavage site for the fibrinogen Aα chain. Of the amino acid sequences discussed, these authors suggested the tripeptide sequence Phe-Val-Arg (P9-P2-P1, hereinafter referred to as the P3-P2-P1 sequence) would be the most effective inhibitor.
Thrombin inhibitors based (at the P1-position of the molecule) upon the 2-heteroaromatic substituted 1-yl-benzylamide structural unit are disclosed in U.S. Pat. No. 7,144,899 and WO2004032834.
Thrombin inhibitors based (at the P2-position of the molecule) upon the 1-acetyl-pyrrolidine-2-carboxylic acid amide, 1-acetyl-piperidine-2-carboxylic acid amide or 1-acetyl-azepane-2-carboxylic acid amide structural units are disclosed in U.S. Pat. No. 7,144,899.
Thrombin inhibitors based (at the P2-position of the molecule) upon the 1-acetyl-pyrrolidine-2-carboxylic acid amide or 1-acetyl-dihydropyrrole-2-carboxylic acid amide structural units are disclosed in U.S. Pat. No. 6,515,011 and WO2004032834.
Thrombin inhibitors based (at the P2-position of the molecule) upon the 1-acetyl-azepane-2-carboxylic acid amide structural unit are disclosed in U.S. Pat. No. 6,528,503.
Thrombin inhibitors based (at the P2-position of the molecule) upon the aza-bicyclo[3.1.0]hexane-1-carboxylic acid amide structural unit are disclosed in U.S. Pat. No. 6,288,077.
Thrombin inhibitors based (at the P2-position of the molecule) upon the 1,3-thiazolidine-2-carboxylic acid amide, 1,3-thiazolidine-4-carboxylic acid amide, pyrazolidine-3-carboxylic acid amide and 4,5-dihydro-1H-pyrazole-5-carboxylic acid amide structural units are disclosed in U.S. Pat. No. 6,740,647 and also described by Lange et al. in Bioorganic & Medicinal Chemistry Letters 16, 2648-2653 (2006).
Quantitative structure activity relationship studies of aryl heterocycle-based thrombin inhibitors are described by Roy et al. in European Journal of Medicinal Chemistry 41, 1339-1346 (2006).
Thrombin inhibitors based (at the P2-position of the molecule) upon 4-fluoroprolines are described by Staas et al. in Bioorganic & Medicinal Chemistry 14, 6900-6916 (2006).
Thrombin inhibitors based (at the P2-position of the molecule) upon pyrazinones carrying various aryl-heterocycles at the P1-position of the molecule are described by Young et al. in Journal of Medicinal Chemistry 47, 2995-3008 (2004).
There remains a need for effective inhibitors of trypsin-like serine proteases, such as thrombin. There is also a need for compounds that have a favourable pharmacokinetic profile. Such compounds would be expected to be useful as anticoagulants and therefore in the therapeutic treatment of thrombosis and related disorders.